Method of electrolytic descaling and pickling

ABSTRACT

METHOD OF DISCALING AND PICKLING TITANIUM AND TITANIUM BASE ALLOY ARTICLES BY IMMERSING THE ARTICLE IN A FIRST ELECTROLYTE CONSISTING OF AN AQUEOUS SOLUTION OF ACID FROM THE GROUP CONSISTING OF PHOSPHORIC ACID AND SULPHURIC ACID WHILE PASSING ELECTRIC CURRENT THROUGH ELECTRODES IMMERSED IN THE ELECTROLYTE AS ANODES AND THROUGH THE ARTICLE AS A CATHODE FOLLOWED BY IMMERSING THE ARTICLE IN A SECOND ELECTROLYTE CONSISTING ESSENTIALLY OF A SOLUTION OF 0.125 TO 1 MOLE PER LIETER OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF SULPHURIC ACID, SODIUM SULPHATE, POTASSIUM SULPHATE AND AMMONIUM SULPHATE, UP TO 0.33 MOLE PER LITER OF A SOLUBLE DICHROMATE, AND 0.5 TO 1.5 MOLES PER LIETER OF HYDROFLUORIC ACID WITH THE BALANCE SUBSTANTIALLY WATER WHILE PASSING ELECTRIC CURRENT THROUGH ELECTRODES IMMERSED IN THE ELECTROLYTE AS CATHODES AND THROUGH THE ARTICLE AS AN ANODE.

United States Patent 3,632,490 METHOD OF ELECTROLYTIC DESCALING AND PICKLING Loren C. Covington, Henderson, Nev., assignor to Titanium Metals Corporation of America, West Caldwell, NJ. No Drawing. Filed Nov. 12, 1968, Ser. No. 775,120 Int. Cl. C231) 1 US. Cl. 204-141 7 Claims ABSTRACT OF THE DISCLOSURE Method of descaling and pickling titanium and titanium base alloy articles by immersing the article in a first electrolyte consisting of an aqueous solution of acid from the group consisting of phosphoric acid and sulphuric acid while passing electric current through electrodes immersed in the electrolyte as anodes and through the article as a cathode followed by immersing the article in a second electrolyte consisting essentially of a solution of 0.125 to 1 mole per liter of a compound selected from the group consisting of sulphuric acid, sodium sulphate, potassium sulphate and ammonium sulphate, up to 0.33 mole per liter of a soluble dichromate, and 0.5 to 1.5 moles per liter of hydrofluoric acid with the balance substantially water while passing electric current through electrodes immersed in the electrolyte as cathodes and through the article as an anode.

This invention relates to a process for descaling and pickling the surfaces of titanium and titanium base alloy articles. More particularly, the invention is directed to a two-step bipolar electrolytic process successively employing different electrolytes for descaling and pickling the article.

My invention provides a rapid and efficient method of descaling and pickling articles of titanium and titanium base alloys. During manufacture, a titanium or a titanium base alloy article is often exposed to relatively high temperature, and oxidation of the metal surface results. This oxidation increases the oxygen content in the surface layer of the metal which is harmful to mechanical properties and may also create an undesirable layer of scale on the surface. The scale consists of fused oxides of titanium and is disadvantageous to further processing and fabrication of the metal article.

Electrolytic descaling and pickling of titanium and titanium base alloy articles wherein the article is immersed in an electrolyte and connected to the positive pole of the source of electric current as the anode is disclosed in United States Patent No. 3,239,440. The electrolyte container may be connected to the negative pole of the source of electric current as the cathode, or a separate cathode may be immersed in the electrolyte. The method disclosed in Patent No. 3,239,440 may be successfully used to treat titanium and certain titanium base alloys but has not been completely successful for treating certain alloys such as those including tin since refractory scale and hot mill scale are not effectively removed from these alloys. The twostep method of the present invention is an improvement over the method set forth in United States Patent No. 3,239,440.

My invention is a multi-step process employing two separate electrolytes. Each electrolyte is contained in a separate tank, and at least one electrode is located in each tank. In the first tank, the electrode is anodic and the article is cathodic as it passes through the electrolyte; and in the second tank, the electrode is cathodic and the article is anodic. Current flows from the electrode in the first container into the cathodic article; and in the second container, it passes from the anodic article through the electrolyte to the cathodic electrodes. The treatment in the first container may be termed cathodic as the article is the cathode, and the treatment in the second container may be termed anodic since the article is the anode.

In the first step of the process where the article is cathodic, hydrogen is vigorously evolved from the metal surface; and this evolution loosens the scale. The vigorous evolution of hydrogen is necessary to loosen the scale, but it is essential that large amounts of hydrogen are not retained by the metal since such is harmful to mechanical properties. When the article passes into the second tank, it becomes anodic; and the surface layer of the metal dissolves in the electrolyte under the influence of the current to remove the scale and the contaminated surface layer of the metal. Removal of the surface layer effectively removes any hydrogen picked up in the cathodic step because, at the temperatures utilized, diffusion is slow and the hydrogen introduced by the cathodic step is largely limited to the surface layers of the metal. Removal of a surface layer from the anodic article requires dissolution of metal which means that a surface layer of titanium oxide must not be formed on the metal. The electrolytes set forth in United States Patent No. 3,239,440 prevent formation of a protective film of titanium oxide on the surface layer of the article when it is anodic and thereby permit dissolution of the metal when current is applied. However, these electrolytes are not successful when the metal article is cathodic; and for this reason, the present process must be carried out with different electrolytes in individual tanks.

The electrolyte used in the cathodic cell must be a good electrical conductor in order to maintain the voltage re quirements at a minimum, and at the same time must be capable of penetrating and softening scale under the influence of electric current.

Descaling and pickling titanium and titanium base alloy articles according to the invention is accomplished by using an aqueous solution of phosphoric or sulphuric acid as the electrolyte in the cathodic cycle and an electrolyte according to United States Patent No. 3,239,440 in the anodic cycle. The anodic cell electrolyte, therefore, consists essentially of from 0.125 to 1 mole per liter of a compound selected from the group consisting of sulphuric acid, sodium sulphate, potassium sulphate and ammonium sulphate, up to 0.33 mole per liter of a soluble dichromate, from 0.5 to 1.5 moles per liter of hydrofluoric acid and the balance substantially water. When phosphoric acid is used as the electrolyte in the cathodic cycle, it may have a concentration between 10% and 70% by volume; although a concentration between about 50% and 70% is preferred since scale removal and conductivity are increased at the higher concentrations. For example, at a temperature of 85 C., a 10% aqueous solution of phosphoric acid will require about 17 volts at 20 amperes; whereas a 70% solution will require only about 6 volts at 20 amperes. Alternatively, when sulphuric acid is used as the electrolyte in the cathodic cycle, it should have a concentration between 30% and 40% by volume. While the conductivity of the solutions of H PO and H is about the same, sulphuric acid is considerably less expensive than phosphoric acid; and a cost advantage is, therefore, obtained by using H 50 However, a considerable amount of fume is evolved when an article is removed from the solution of H 80 and for this reason, it may be advantageous to use the more expensive phosphoric acid solution and eliminate the harmful fumes.

The cathodic cycle may be carried out at any temperature down to room temperature; but I have found that this cycle is most efficient if the temperature is maintained just below the boiling point of the electrolyte, as descaling action is better and the conductivity of the electrolyte is increased The current density in the cathodic cycle should be maintained between about 100 and 1000 amps/ft. since hydrogen evolution is not large enough to effectively remove scale at current densities below 100 amps/ft. and at current densities above 1000 amps/ft. the action of the electrolyte is so vigorous that it may boil out of the tank.

In the anodic cycle, it is preferred that the initial temperature of the electrolyte not exceed about 80 C. since loss of HF and water due to evaporation is excessive above 80 C. However, the efiiciency of the reaction appears to be unimpaired up to the boiling point of the electrolyte. It is also preferred that the final temperature of the electrolyte be above about 50 C. since the conductivity of the electrolyte decreases rapidly at low temperatures, and voltage requirements are correspondingly large. Pickling will, however, proceed at room temperature, although the voltage requirements are so large as to make room temperature operation impractical.

The current density in the anodic cycle should be between about 80 and 1000 amps/n. with the reaction being more vigorous at higher values. At current densities below about 80 amps/f, the chemical action of the electrolyte will predominate and undesirable hydrogen evolution will commence on the surface of the anodic article.

The anodes in the cathodic cell are preferably either graphite or platinum-plated titanium, and the cathodes in the anodic cell may be stainless steel. Stainless steel anodes may also be used, but such is not preferred because the anode material goes into solution and contaminates the electrolyte requiring frequent replacement with fresh electrolyte.

The following non-limiting examples indicate the prac tice of my invention.

EXAMPLE 1 A specimen of Ti--Al--2.5Sn covered with heavy hot roll mill scale was made cathodic in a 70% aqueous solution of H PO at 95 C. for minutes while a current density of 288 amps/ft. was maintained. The resulting specimen had a darkened surface but was largely free of scale.

The specimen was next made anodic in a solution of 33 g./1. (NI-10 80, 25 g./l. Na Cr O and 2S mL/l. of 48% HF for 5 minutes at a current density of 200 amps/fe The solution was at room temperature initially and increased to about 65 C. by the end of the cycle.

The specimen was then rinsed in water, and the surface was bright and clean and all scale had been removed.

EXAMPLE 2 A specimen of Ti75A 1 in. by 6 in. was cleaned and placed in a muflie furnace at 1450" F. for 30 minutes to simulate an annealing cycle. A dense scale was formed on the surface of the specimen.

The specimen was then made cathodic in a 35% aqueous solution of H 80 solution for 10 minutes at a current density of 168 amps/ft. and a temperature of 85 C. At the end of this cycle, the specimen was completely descaled but the surface was a dull grey.

The specimen was then made anodic in a solution having the composition of Example 1 for 5 minutes at a current density of 168 amps/ft? The soultion was at room temperature initially and increased as in Example 1.

At the end of the anodic cycle, the specimen was bright and clean and completely free of scale.

EXAMPLE 3 Four specimens of T i5Al2.5Sn having a hard yellow hot roll scale and having a surface area of 10 in. were subjected to various treatments.

The first three specimens were made cathodic for 10 minutes at a current density of 288 amps/ft? Specimens Nos. 1 and 2 were processed in an electrolyte consisting of a 70% aqueous solution of H PO at 95 C., and specimen No. 3 was processed in a 35% aqueous solution of H 50 at C. Specimen No. 4 was not given a cathodic cycle.

All four specimens were then subjected to the anodic cycle for 5 minutes at a current density of 200 amps/ft. in the same electrolyte as in Example 1.

The first three specimens were completely descaled and had a clean surface; whereas specimen No. 4, which had not been subjected to the cathodic treatment, was not descaled. This shows that descaling of tin-containing titanium alloys can be achieved utilizing the process of the instant application; whereas an anodic treatment will not descale tin-containing titanium base alloys.

EXAMPLE 4 Six specimens 1 in. x 6 in. were considered in this example. Two specimens were Til3Vl lCr-3Al; two specimens were Ti8Al1MolV; and two specimens were Ti5Al-2.5Sn. Each specimen was exposed to a temperature of 1450 F. for 30 minutes to form anneal scale on the surfaces thereof.

Each of the specimens was treated in the cathodic cycle for 10 minutes at a temperature of 95 C. and a current density of 316 amps/ft The electrolyte in the cathodic cycle was a 70% aqueous solution of H PO Each specimen was then treated in the anodic cycle for 5 minutes at 200 amps/ft. during which the temperature of the electrolyte increased from room temperature to about 65 C. The electrolyte used for the anodic cycle was the same as in Example 1.

All six of the specimens were completely descaled.

EXAMPLE 5 A specimen of Ti8Al1MolV 0.1 in. thick and having a surface area of 12 in? having hot roll mill scale on the surface was treated cathodically for 10 minutes at a current density of 200 amps/ft. in a 70% aqueous solution of H PO at C.

The specimen was then treated anodically for 10 minutes in the same electrolyte as in Example 1 at a current density of 200 amps/ft? Complete descaling was obtained and the surface was bright and clean.

Since it is known that the hydrogen evolved from the electrolyte at the surface of an article of titanium or titanium base alloy when connected as the cathode is readily absorbed, a number of specimens having 0.003% hydrogen were processed according to my invention. The specimens were analyzed for hydrogen before treatment and after being treated according to the method of my invention. The hydrogen content of the final specimens varied from 0.0032% to 0.005% which is not harmful to the ductility of the metal. The analyses showed an average hydrogen pickup during the cathodic cycle of about 0. 0095% and an average decrease during the anodic cycle of about 0.0085%. Therefore, the total average hydrogen pickup during the descaling and pickling of the specimens was only 0.001%. It is apparent that the hydrogen picked up during the cathodic cycle is substantially removed during the anodic cycle by removal of the surface layer of the metal.

The two-step process of my invention is a significant improvement over the single step process disclosed in United States Patent No. 3,239,440 as it may be effectively used with titanium and titanium base alloys including those including those containing tin as an alloying element. The process is capable of removing the most diificult scales encountered on titanium and titanium base alloys and does not add a significant amount of hydrogen to the metal.

While I have shown and described preferred embodiments of my invention, it may be otherwise embodied within the scope of the appended claims.

I claim:

1. Method of descaling and pickling an article from the group consisting of titanium and titanium base alloys after said article has been annealed or hot rolled, said method comprising: immersing said article in a first aqueous acid solution electrolyte selected from the group consisting of H PO and H 80 passing electric current between said article as a cathode and electrodes immersed in said first electrolyte as anodes to evolve hydrogen at the surface of said article and thereby loosen scale adhering to the surface of said article, removing said article from said first electrolyte and immersing said article in a second electrolyte comprising a solution of 0125-1 mole per liter of a compound selected from the group consisting of sulphuric acid, sodium sulphate, potassium sulphate and ammonium sulphate, up to 0.33 mole per liter of a soluble dichromate, and 0.51.5 moles per liter of HF with the balance substantially Water passing electric current through said article as an anode and through electrodes immersed in said second electrolyte as cathodes to dissolve the surface layer of said article to remove scale and hydrogen contaminated metal and removing said article from said second electrolyte, whereby anneal scale and hot mill scale are removed from the surface of said article and the surface of said article is bright and clean.

2. The method edscribed in claim 1 wherein said first electrolyte is an aqueous solution of H PO having a concentration between and 70% by volume.

3. The method described in claim 2 wherein said aqueout solution of H PO has a concentration between 50% and 70% by volume.

4. The method described in claim 1 wherein said first electrolyte is an aqueous solution of H 80 having a concentration between 30% and 40% by volume.

5. The method as described in claim 1 wherein the temperature of said first electrolyte is maintained below the boiling point of said first electrolyte and the temperature of said second electrolyte is maintained between about C. and about C.

6. The method as described in claim 1 wherein said electric current is passed between said electrodes and said article in said first electrolyte at a current density between about and 1000 amps/ft. and between said electrodes and said article in said second electrolyte at a current density between about 80 and 1000 amps/ft 7. The method as described in claim 1 wherein said first electrolyte is an aqueous solution of H PO having a concentration of about 50% by volume and wherein said current is passed between said electrodes and said article for a period of about 10 minutes.

References Cited UNITED STATES PATENTS 2,780,594 2/1957 Dailey 204-141 3,006,827 10/1961 Capuano 20414l 3,030,286 4/1962 Tao 204-141 3,239,440 3/1966 Covington 20414l OTHER REFERENCES Colner et al.: Electroplating on Titanium, Journal of Electrochemical Society, November 1953, vol. 100, pp. 485489.

JOHN H. MACK, Primary Examiner N. A. KAPLAN, Assistant Examiner US. Cl. X.R. 204-32 

